Matt. 28:19-20 "Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen."
God is a tri-unity. This categorical statement may appear illogical to any reader who is confronted for the first time with this truth. The foregoing discussion makes such an effort to emphasize the unity of God, that the reader is now surprised to discover something about God that seems to be in contradiction. At first glance, the title "tri-unity" may appear to be a mistake, since the more common term is "trinity." But the expression "trinity" lays emphasis upon the three persons in the Godhead to the exclusion of the one essence, whereas the word "tri-unity" includes both ideas, and therefore insists upon a trinity of persons in the one divine essence.
The importance of this divine reality dare not be underestimated. Of all the propositions relating to God, this one is the fundamental of the fundamentals of the Christian faith. This differentiates the Christian faith from all other faiths. Without this doctrine there could be no divine Savior and no divine Holy Spirit. Without this truth there could be no incarnation of God in human flesh. There could be no transaction between persons of the Godhead in atonement. And there could be no God of eternal love. It follows that all the distinctive truths of Christianity are wrapped up in this great fact. "Without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory" (1 Tim. 3:16).
When Christ became flesh, the tri-unity of God came into full view. In the doctrine of God, tri-unity comes last. This marks progress in revelation. This progress is not from imperfection to perfection, for at every stage in revelation the body of truth was perfect, though at that point it was incomplete. God revealed truth in the same proportion that men were prepared and able to receive it. When at last Christ came, men were then amply prepared and able to receive the crowning truth in the doctrine of God, the tri-unity of God. But this does not mean that there is no revelation concerning the tri-unity of God in the Old Testament. The facts are that this great truth is woven into the very warp and woof of the Old Testament Scriptures, except for the fact that it is not clearly seen because men are not sufficiently prepared to recognize it. The Hebrew word for God in the opening verse of Genesis is plural in number, though it is used with a singular verb. This occurs over and over again in the Old Testament. Notice the use of the word "us" in connection with the creation of man (Gen. 1:26), and his expulsion from the garden (Gen. 3:22). It would appear that the issues were so important that there was need for divine consultation in the Godhead. The answer to the critics of this doctrine is that the complete revelation of the tri-unity of God was given in deeds rather than words. Godly Jews believed in a God who dwelt in heaven. When Christ came, these very Jews recognized Him as God. The coming of the Holy Spirit on Pentecost was recognized as something different from the Father or from Christ, and when these men wrote the New Testament there was no conscious effort to formulate a doctrine of the tri-unity of God. All they did was to record what happened. "O the depths of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out" (Rom. 11:33). To seal this truth to each believer at the outset of his experience of salvation, he is baptized "into the name of the Father and of the Son and of the Holy Spirit" (Matt. 28:19), a trinity of divine persons all contributing to his salvation.
Both water and spirit baptism in John 3:5-6.